The present invention holds forth improvements in the field of nanotechnology, particularly as relates to the production of the method of growing lead chalcogenide nanocrystals from the surface of titanium dioxide in organic solvents.
Titanium dioxide is an important photovoltaic and photocatalytic material1, which utilization in dye-sensitized solar cells2, and hydrogen production3 is encouraged by its low fabrication costs and minimal environmental hazards. Efficient harvesting of solar radiation within TiO2 generally requires extending its absorption range into the visible and near infrared by introducing an appropriate sensitizer that engages in electron-transfer reaction with an oxide material upon receiving a photon of light. To date, the most common strategy for the sensitization of TiO2 involves modification of its surface with organic-based transition metal complexes, such as porphyrins4 or Ru-complexes2. On the other hand, incorporation of semiconductor nanocrystals (NC) as sensitizers is now being actively explored5-20 due to a number of advantages offered by inorganic NCs, including wider absorption profile, superior resistance to photobleaching and continuous tunability of NC conduction levels.
As shown by recent reports, successful modification of anatase TiO2 or amorphous TiOx with colloidal CdSe, InAs21, PbSe22, and PbS23 NCs can be achieved in a reproducible manner leading to heterostructures that exhibit photoinduced charge separation. In these works, however, deposition of colloidal nanocrystals onto the oxide material still relies on organic linkers or non-epitaxial contacts with NC ligands, which makes it difficult to extract photoinduced carriers from NC domains leading to the decrease in electron transfer rates and carrier trapping at hybrid interfaces. For instance, surprisingly slow photo-induced electron transfer has been reported in organically coupled PbS—TiO2 systems. To avoid these problems, several groups have attempted in situ growth of CdS24,25,26 NCs onto mesoporous TiO2 films in ionic solutions. While the observation of the 2-3 fold increase in the solar conversion efficiency of such films was encouraging, the quality and size-distribution of NCs fabricated using this approach was inferior to those synthesized through colloidal techniques, making it difficult to control relative positions of electron energy levels in a donor-acceptor system.
The present invention provides a colloidal route to the synthesis of PbSe/TiO2 hetero-nanocrystals (HNCs), comprising 2-5 nm PbSe NCs grown directly on the surface of TiO2 nanorods (NRs). As a main benefit of colloidal injection techniques, the present approach allows for a controlled adjustment of the sensitizer electronic levels via tuning the average NC diameter during synthesis, which is critical for the experimental realization of a desired type II (staggered) offset of donor and acceptor conduction band edges. Moreover, formation of a near-epitaxial interface between PbSe and TiO2 domains enables a rapid injection of photoinduced carriers into the oxide material, which was demonstrated via a 50-fold increase in the photoinduced electron transfer rate between PbSe and TiO2 domains, as compared to organically linked lead chalcogenide-TiO2 assemblies.
The present invention thus provides a colloidal synthesis of PbSe/TiO2 heterostructures, comprising small-diameter PbSe nanocrystals epitaxially grown onto the surface of TiO2 nanorods. The deposition of lead sulfide onto prefabricated TiO2 nanocrystals proceeds via formation of a thin PbSe shell that subsequently breaks into sub-2-nm islands. Additional precursor injections are then used to increase the size of PbSe nanocrystals up to 5 nm. In the case of small-size PbSe, a 2.1-ns transfer of photoinduced carriers into TiO2 domain was evidenced through quenching of the PbSe band-gap emission. Overall, present synthesis demonstrates a colloidal approach to all-inorganic modification of TiO2 surfaces with semiconductor nanocrystals, which provides a viable alternative to a more common supramolecular assembly of nanocrystal-oxide composites.
In addition, semiconductor nanocrystals (NCs) are considered to be a promising class of materials for the development of new-generation solar cells due to the superior optoelectronic properties of inorganic semiconductors and the stability of as-prepared nanocrystals in colloidal suspensions, which allows for inexpensive, solution-based device fabrication. The demonstrated potential of these materials for the enhanced absorption of solar light and efficient spatial separation of photoinduced carriers have given rise to several photovoltaic architectures, including hybrid bulk heterojunction, all-inorganic, and nanocrystal-sensitized solar cells. For instance, spincoated films of semiconductor NCs can be introduced as light-absorbing layers and simultaneously as n-type components in nanocrystal/polymer bulk heterojunction solar cells, or, alternatively, incorporated into an all-inorganic bi-layer design comprising a heterojunction of n- and p-type NC films. The use of semiconductor NCs in third-generation light-harvesting applications has greatly improved the device resistance against oxidation and photodegradation, but, so far, has not yet resulted in desired power conversion efficiencies. In particular, the main weakness of photovoltaic architectures that rely on colloidal NCs remains to be a rather poor carrier collection at the working electrode, which is caused by inefficient intra-particle conductance of photogenerated charges through a three-dimensional matrix of organically coated nanocrystals.27-32 
The problem of poor carrier collection in NC-based solar cells can be avoided if colloidal NCs are excluded from the process of carrier transport, and participate primarily as inorganic light-absorbing elements. This type of light-harvesting mechanism has been successfully implemented in the past through a dye-sensitized solar cell design2, where semiconductor NCs were employed as light sensitizers in lieu of organic dyes. The transport of photogenerated electrons towards a working electrode, in this case, is mediated via mesoporous TiO2, while photoinduced holes are regenerated by means of a liquid33,34,35 or solid36,37 electrolyte. From the long-term prospective, the attempted replacement of organic sensitizers with inorganic semiconductor NCs in dye-sensitized solar cells constitutes a natural step towards increasing the intake of solar energy and improving the system's long-term stability against photodegradation, but may require further development of the donor-acceptor architecture, as was highlighted by a number of recent investigations38-51.
At present, there are two main schemes for incorporating NC sensitizers onto the surface of TiO2, which include (i)—introducing organic linker-molecules that bridge colloidal NCs with TiO2, and (ii)—growing NCs directly onto the oxide surface via chemical bath deposition (CBD) or successive ionic layer adsorption and reaction (SILAR) process52. The former approach has been successfully employed to couple CdS53, CdSe45,54,55, InAs, PbSe56 and PbS57 NCs onto TiO2 via 3-mercaptopropionic acid (MPA) or through a non-epitaxial contact to organic ligands and, thus far, has yielded up to 1.7% of power conversion efficiency (PCE). The main drawback of this method is considered to be the presence of supramolecular organic spacers between nanocrystal and oxide domains, which augment the tunneling barrier between excited states of a sensitizer and the conduction band of TiO2, causing a decrease in electron transfer probability. In addition, a number of experimental works have demonstrated that organic linkers can also serve as carrier traps, which further reduce the electron transfer rate. These drawbacks, associated mainly with the presence of organic linkers in the NC-TiO2 assembly, are successfully avoided in the second deposition method, where NCs are grown directly onto the TiO2 via chemical bath deposition CBD18,40,41,50,58,59,60, leading to all-inorganic NC/TiO2 heterostructures. Indeed, recent studies have shown that CBD-grown NC/oxides films show an improved carrier injection and superior PCE and Incident Photon to Current Efficiency (IPCE) values61. The quality and the size distribution of CBD grown NC sensitizers, however, remain inferior to those synthesized through colloidal techniques62. Furthermore, the aqueous route to growing of NC sensitizers through CBD approach is often ineffective with air-sensitive semiconductors, such as PbSe or PbS, revealed by an overall low crystallinity and poor optical properties.
The present invention provides a relatively facile method for developing PbS-sensitized TiO2 films, which combines the benefits of the hot-injection colloidal route to the synthesis of monodisperse PbS NC sensitizers, with the advantages of linker-free, all-inorganic, PbS/TiO2 heterojunction. In the present approach, the processes of sensitizer deposition and sintering of the film are performed in the reverse chronological order, which allows for a hot-temperature growth of the PbS sensitizer directly onto the surface of TiO2, followed by a solution-phase deposition of PbS-modified TiO2 nanoparticles onto a conductive substrate. The resulting PbS/TiO2 films are then subjected to high-temperature annealing process to remove residual surface ligands, whereby yielding an all-inorganic absorbing layer. In view of the renewed interest in NC-sensitized solar cells, which revival is fueled by the on-going research and discovery of non-corrosive36,63,64, and longer-lasting35 hole-scavenging materials, the PbS/TiO2 heterostructured films, reported here, could potentially lead to high-efficiency NC-sensitized solar cells.
Recently, colloidal growth of hetero-epitaxial semiconductor/TiO2 composites via hot-injection routes has been demonstrated by several groups.65,66 Acharya et. al.66 has reported the synthesis of PbSe/TiO2 nanostructures, comprising epitaxial assemblies of PbSe NCs and TiO2 nanorods, where the average size of NC sensitizer could be tuned from 2 to 5 nm. Unfortunately, a relatively low position of the conduction band edge in PbSe NCs, prevented the desirable combination of a narrow-band-gap PbS sensitizer and proper alignment of PbSe and TiO2 excited state levels needed for an efficient electron transfer. In contrast to PbSe/TiO2 heterostructures, the PbS/TiO2 combination of materials, explored in accordance with the present invention, supports the PbS to TiO2 electron transfer even for large diameter PbS NCs57 (d≦7 nm), which allows obtaining a near-optimum band gap for the solar energy conversion (ℏω≈1.4 eV). The present invention provides a controllable tuning of PbS domain sizes in the 2-15 nm range with an average dispersion of PbS diameters between 9% and 14%. Owing to a sequential two-step approach to the synthesis of TiO2/PbS NCs, the size and the shape of TiO2 domains can be tuned as well, which provides an additional degree of freedom for optimizing the transport of photoinduced carriers through an array of TiO2/PbS nanoparticles.
In accordance with the preferred embodiment of the present invention, following purification, colloidal suspensions of synthesized PbS/TiO2 NCs may be used to fabricate ethanol-based pastes for the development of thin films on top of a conductive transparent electrode. After annealing, these films were incorporated into a standard two-electrode cell filled with polysulfide electrolyte for electrochemical characterization. The measured photoaction spectra were found to compare favorably with those of PbS-sensitized TiO2 films fabricated via conventional chemical bath deposition route.
The invention comprises the method of growing lead chalcogenide nanocrystals from the surface of titanium dioxide in organic solvents, lead chalcogenide/TiO2 nanocomposites colloids produced by the claimed method, and the application of lead chalcogenide/TiO2 nanostructures as an active absorbing element in nanocrystal-sensitized solar cells.
The present invention includes compositions of matter and methods of their synthesis, the compositions including a new class of nanocomposite colloids.
The method of the present invention may be understood as a method of preparing a lead chalcogenide/TiO2 nanocomposite colloid, the method comprising injecting the lead and chalcogenide precursors of the lead chalcogenide/TiO2 nanocomposite colloid into a heated mixture of: (a) organically passivated TiO2 nanoparticles; and (b) at least two surfactants capable of promoting the growth of lead chalcogenide nanocrystals directly onto the surface of the TiO2 nanoparticles, the at least two surfactants being present in a ratio that may be changed, whereby the shape of the nanocrystals formed in the mixture of surfactants is capable of being controlled by adjusting the ratio of the lead chalcogenide surfactants in the binary mixture.
It is preferred that the mixture comprises lead chalcogenide domains selected from the group consisting of PbS, PbSe, PbTe, PbSxSe1−x nanocrystals, and mixtures thereof.
The TiO2 nanoparticles may have shapes selected from the group consisting of nanorods, tetrapods, and spheres, and mixtures thereof. Preferably, the TiO2 nanoparticles have shapes selected from the group consisting of nanorods, tetrapods, spheres, and mixtures thereof, wherein the smallest dimension size thereof is in the range of from about 2 to about 10 nm, and wherein the largest dimension size thereof is in the range of from about 5 to about 100 nm. The surfactants that may be used in the present invention may include any appropriate surfactant consistent with the formation of colloids of the type of the present invention, such as those selected from the group consisting of acids, amines, alkanes, and mixtures thereof.
The present invention also includes a lead chalcogenide/TiO2 nanocomposite fabricated by the process according to present invention. It is preferred that the lead chalcogenide/TiO2 nanocomposite is soluble in at least one organic non-polar solvent.
It is also preferred that the lead chalcogenide/TiO2 nanocomposite has lead chalcogenide domain sizes in the range of from about 1.5 to about 10 nm. It is further preferred that the lead chalcogenide/TiO2 nanocomposites comprise multiple lead chalcogenide domains adjacent to a single TiO2 nanoparticle host. Also preferred is that the lead chalcogenide/TiO2 nanocomposite having TiO2 domains having shapes selected from the group consisting of nanorods, tetrapods, and spheres, and mixtures thereof, and having a smallest dimension in the range of from about 2 to about 10 nm, and a largest dimension in the range of from about 5 to about 100 nm.
The method of the present invention also includes a method of preparing a lead chalcogenide/TiO2 nanocomposite colloid, the method comprising: (a) injecting the lead and chalcogenide precursors of said lead chalcogenide/TiO2 nanocomposite colloid into a first heated mixture of: (1) organically passivated TiO2 nanoparticles; and (2) at least two surfactants present in the reaction mixture during lead chalcogenide deposition step capable of promoting the growth of lead chalcogenide nanocrystals directly onto the surface of said TiO2 nanoparticles, said at least two lead chalcogenide surfactants being present in a first ratio; and (3) allowing lead chalcogenide nanocrystals directly onto the surface of said TiO2 from said first heated mixture; and (b) preparing a second heated mixture of: (1) organically passivated TiO2 nanoparticles; and (2) at least two surfactants present in the reaction mixture during lead chalcogenide deposition step capable of promoting the growth of lead chalcogenide nanocrystals directly onto the surface of said TiO2 nanoparticles, said at least two lead chalcogenide surfactants being present in a second ratio, (3) allowing lead chalcogenide nanocrystals directly onto the surface of said TiO2 from said second heated mixture, whereby the shape of said nanocrystals formed in said second mixture is different than the shape of said nanocrystals formed in said first mixture.
The method of the present invention also includes a method of a method of preparing lead chalcogenide nanocrystals of a desired shape from a lead chalcogenide/TiO2 nanocomposite colloid, the method comprising injecting the lead and chalcogenide precursors of said lead chalcogenide/TiO2 nanocomposite colloid into a heated mixture of: (a) determining the desire shape of lead chalcogenide nanocrystals to be prepared; and (b) injecting the lead and chalcogenide precursors of said lead chalcogenide/TiO2 nanocomposite colloid into a heated mixture of: (1) organically passivated TiO2 nanoparticles; and (2) at least two surfactants present in the reaction mixture during lead chalcogenide deposition step capable of promoting the growth of lead chalcogenide nanocrystals directly onto the surface of said TiO2 nanoparticles, said at least two lead chalcogenide surfactants being present in a ratio selected to said nanocrystals to be formed in said desired shape; and (c) allowing lead chalcogenide nanocrystals directly onto the surface of said TiO2 from said heated mixture.
The present invention also includes a nanocrystal-sensitized solar cell having a conductive transparent electrode, and comprising lead chalcogenide/TiO2 nanocomposites such as those produced by the method of the present invention, wherein the nanocomposites act as an active absorbing element and are deposited in form of a paste onto the conductive transparent electrode. The present invention also includes a method of gathering solar energy using the nanocomposites of the present invention.
The present invention also includes a method of gathering solar energy using the nanocomposites of the present invention.
The present invention also includes a method of producing hydrogen using the nanocomposites of the present invention.
In another variation of the present invention, the present invention provides hetero-epitaxial growth of nearly monodisperse PbS nanocrystals onto the surface of TiO2 nanoparticles via colloidal hot-injection routes (i.e., routes in excess of 100 degrees C.; typically about 180 C). Fabricated PbS/TiO2 nano-composites can be dispersed in non-polar solvents, which enables an easy solution processing of these materials into mesoporous films for use as light-absorbing layers in nanocrystal-sensitized solar cells. High-temperature deposition of the sensitizer material allows controlling both the size and the number of PbS domains that are grown onto TiO2 nanoparticles, whereby providing synthetic means for tuning the spectral sensitivity of PbS/TiO2 nano-composites and simultaneously enhancing their photocatalytic response in the visible and near-infrared. Compared to conventional ionic bath deposition of PbS semiconductors onto TiO2, the results obtained in accordance with the method of the present invention show an improved nanocrystal quality and narrower distribution of PbS sizes, thus allowing for precise tuning of excited state energies in a PbS—TiO2 donor-acceptor system, meanwhile, the use of hot-temperature deposition of PbS (T=180° C.) promotes the formation of epitaxial relationships between PbS and TiO2 domains, leading to fewer interfacial defects. The photovoltaic response of pyridine-treated PbS/TiO2 nano-composites was measured in a two-electrode configuration using polysulfide electrolyte. The measured photoresponse compares favorably to that of PbS-modified TiO2 electrodes fabricated via chemical bath deposition.
The characteristics and uses of lead chalcogenide are also described for instance in U.S. Published Patent Applications Nos. 20090293928, 20090251759, 20090178700, 20090162278, 20080296534, 20080224121, 20080057311, 20070090336, 20060110313, and 20050120946, as well as in U.S. Pat. Nos. 5,028,563, 4,943,971, 4,743,949, 4,709,252, 4,608,694, 4,477,730, 4,442,446, 4,413,343, 4,371,232, 4,282,045, 4,263,604, 4,227,948, and 4,126,732, all of which are incorporated herein by reference. These use and devices may be considered to be part of the present invention to the extent lead chalcogenide produced in accordance with the present invention may be applied to such uses, devices and methods.
The present invention also includes